Keizo Kobayashi

Development and Evaluation of High-Strength Fe2VAl Thermoelectric Module

A thermoelectric module was constructed using p-type Fe2V0.9Ti0.1Al and n-type Fe2VAl0.9Si0.1 sintered alloys prepared by pulse-current sintering (PCS). The thermoelectric power factors of the p- and n-type materials were 1.6 and 4.1 mW/mK2, respectively, at 350 K. The bending fracture strength of the sintered alloy was 800 MPa and was 10 times higher than that of the Bi–Te alloy. These sintered alloys were connected in series with a copper electrode. The connection was induced by Joule heating at a high pressure without electrically conductive jointing materials, such as solder and Ag paste. The shear strength of the direct bonding was estimated to be 100 MPa. The electrical resistance of the bonding was negligibly small compared with that of the alloy. The estimated power density of the conventional pi-type unicouple was 378 mW/cm2. The maximum output power of the thermoelectric module consisting of 18 pairs of p–n junction was estimated to be 1.2 W on a hot plate at 573 K in air.

Synthesis of nanocrystallite by mechanical alloying and in situ observation of their combustion phase transformation in Al3Ti

Elemental powders of stoichiometric Al3Ti were mechanically alloyed (MA) in order to investigate the phase formation during the milling process. Furthermore the stability of MA powders were studied under transmission electron microscopy (TEM). The results indicate that a supersaturated Al(Ti) solid solution with nanocrystalline size has been formed after mechanical alloying for 360 ks in consuming the elemental powders of Al and Ti and no further phase transformation can be detected upon longer milling. The MA powders are unstable being irradiated by electron beams under the TEM observation, exothermically forming various intermetallic compounds. The combustion phase transformation processes and products are depending on the time of mechanical alloying. The structural changes and phase transformations during both mechanical alloying process and annealing process were also characterized by using X-ray diffraction measuring.

Properties of New TiC/TiB2/Fe-Al Cermet Alloy

Effect of TiB2 substitution on thermal conductivity and hardness in TiC / Fe-Al cermets was investigated. The (70-x)TiC / xTiB2 / 26Fe-4Al mass % cermets were fabricated by mechanical milling and subsequent pulsed current sintering method. The high relative density compacts was formed by sintering at 1423 K under 25 MPa for 60 s. The sintered materials were mainly composed of TiC, TiB2 and Fe-Al intermetallic compound. In addition, small amounts of Fe2B surrounding TiB2 were formed. The thermal conductivity of the sintered compact lineally increased with increasing TiB2 volume fraction. However, the hardness of the sintered compacts of x = 20 – 40 were higher than that of x = 0. Therefore, the substitution of TiC to TiB2 in the TiC / Fe-Al based cermet is effective to improve the thermal conductivity without the degradation of hardness.

Mechanical Alloying of Ti-Nb-Si and Their Consolidation by Pulsed Current Sintering

Ti-xat%Nb-10at%Si (x=5-40) alloys were synthesized by mechanical alloying (MA) using a planetary ball mill with as starting materials of Ti, Nb and Si powders. The MA powders milled for 360ks were spherical particles of about 20μm in all the alloys studied. The characteristics of these MA powders were examined by XRD, DSC and TEM.Ti-5at%Nb-10at%Si powder milled for 360ks consisted of fine α-Ti crystals. The MA powders containing 10at%Nb to 40at%Nb showed exothermic reaction in the DSC curves. In Ti-20at%Nb-10at%Si and Ti-10at%Nb-10at%Si, the exothermic reactions were observed at higher temperature than 800K. This exothermic reaction was the crystallization from amorphous phase in MA powder to α-Ti crystalline phase. In Ti-40at%Nb-10at%Si and Ti-30at%Nb-10at%Si, the exothermic reaction was observed at about 600K. This exothermic reaction was characterized as crystallization from amorphous phase to β-Ti crystalline. The Ti-10at%Nb-10at%Si MA powder was consolidated by the pulsed current sintering with a high pressure. The obtained bulk amorphous compact contained 18vol% pores.